Nonlinearities in power amplifier performance can harm performance. In the case of wireless networks, nonlinearities in the amplifiers of network stations, e.g., cellular base stations, can negatively affect the performance of the wireless network being served by the network station. One problem that can be caused by power amplifier nonlinearity is the creation of adjacent channel interference in a wireless communication network. The power amplifier is typically the most nonlinear part of wireless transmitters and its nonlinearity can lead to generation of interference in adjacent frequency channels, as characterized by the adjacent channel power ratio (ACPR), and loss of signal accuracy, as characterized by the error vector magnitude (EVM). Accurate amplifier nonlinearity characterization is a key step for amplifier linearization synthesis. Receiver nodes can have difficulty decoding communication packets when levels of adjacent channel interference are high.
Bench top and periodic tests are currently done using test pulses. However, variances in amplifier performance can be introduced by heat and age of the circuitry. Bench top tests thus fail to account for non-linearity that can occur during the operational life of a particular amplifier. Periodic tests may catch life time operational changes, and amplifier compensations may be implemented. However, periodic tests interrupt the normal operation of amplifiers, and degraded performance can occur during a period between periodic tests. Also, periodic and bench tests use test signals that may not reveal all non-linearities.
Conventional single tone or two-tone testing is widely used to extract the amplifier nonlinearity from AM-AM and AM-PM distortion or intermodulation distortion measurements. See, e.g., C. J. Clark et al., “Power-amplifier characterization using a two-tone measurement technique,” IEEE Trans. Microwave Theory Tech., vol. 48, vol 50, no. 6, pp. 1590-1602, (June 2002); A. M. Saleh, “Frequency-independent and frequency-dependent nonlinear models of TWT amplifiers,” IEEE Trans. Comm., vol 29, no. 11, pp. 1715-1720, (November 1981); H. Ku et al., “Extraction of accurate behavioral models for power amplifiers with memory effects using two tone measurements,” in IEEE MTT-S S Int. Microwave Symp. Dig., vol. 1, pp. 139-142 (June 2002). Employing such measurements, the spectral regrowth and constellation distortions of complex digital modulated signals can be predicted by various behavioral PA models. See, e.g., S. W. Chen et al, “Effects of nonlinear distortion on CDMA communications Systems,” IEEE Trans. Microwave Theory Tech., vol. 44, no. 12, pp. 2743-2750, December 1996; .G. T. Zhou and J. S. Kenney, “Predicting spectral regrowth of nonlinear power amplifiers,” IEEE Trans. Commun., vol 50, no. 5, pp. 718-722, May. 2002; K. Gard et al, “Generalized autocorrelations of spectral regrowth from bandpass nonlinear circuits,” in IEEE MTT-S Int. Microwave Symp. Dig., vol. 1, June 2001, pp. 9-12. However, these sinusoidal tone(s) approaches require special input signals and are generally not applicable for characterization of amplifiers during real-time operation.
Alternate adaptive methods with actual code-division-multiple-access (CDMA) signals have been investigated in both time and frequency domains. While such methods have the advantage of being implemented during amplifier operation, it imposes a high computational burden and requires complex hardware. For example, others have proposed addressing amplifier linearity by recording the output of the amplifier over a suitable period of time, and comparing it with the corresponding input signal after time alignment and scaling. See, e.g., S. P. Stapleton, et al., “Simulation and analysis of an adaptive predistorter utilizing a complex spectral convolution,” IEEE Trans. Microwave Theory Tech., vol. 41, no. 4, pp. 387-393, (November 1992); S. Boumaiza & F. M. Ghannouchi, “Realistic power-amplifiers characterization with application to baseband digital predistortion for 3 G base stations,” IEEE Trans. Microwave Theory Tech., vol 50, no. 12, pp. 3016-3021, (December 2002). The power amplifier output spectrum can be calculated by the fast Fourier transform (FFT) under operating conditions. This approach is often used in adaptive digital/RF predistortion and provides good linearization performance. However, high speed and high-resolution analog to digital converters (ADCs) and intensive digital signal processing (DSP) operation are usually required. Another approach that has been proposed is to directly analyze the spectrum components of output power from amplifiers with a simplified spectrum-analyzer-like architecture. S. Lee, et al., “An adaptive predistortion RF power amplifier with a spectrum monitor for multicarrier WCDMA applications,” IEEE Trans. Microwave Theory Tech., vol 53, no. 2, pp. 786-793, (February 2005). With these techniques, hardware complexity is still quite high and hardware components have accuracy requirements.